In gasoline engines, catalytic after-treatment of exhaust gases by means of a controlled three-way catalytic converter has become the established method for complying with the legal requirements on emission limitation. The complete combustion of fuel is found to require approximately 14.7 kg of air per kg of fuel. For the purpose of characterising the composition of the mixture, the air ratio λ is defined as the ratio of the quantity of air supplied for combustion of a unit quantity of the supplied fuel to the minimum quantity of air required for complete combustion.
The air-fuel mixture is controlled by means of a lambda probe, enabling a conclusion to be drawn about the lambda value of the exhaust gases. In the case of zirconium dioxide probes a step characteristic for lambda=1 occurs. Each time the mixture changes from rich to lean or from lean to rich a voltage jump occurs. The combined effects of ageing and environmental influences (contamination) on the catalytic converter give rise to defects which displace the voltage jump. For this reason a controller with a two-point reference probe is used downstream of the catalytic converter, such that the probe downstream of the catalytic converter changes the controlled shift between rich and lean by means of a slow compensation loop.
In addition to the two-point controller described previously, a controller is also known which uses probes described as broadband.
These deliver an unequivocal signal in a wide lambda range 0.7<λ<4 which increases in linear fashion. Such a steady probe enables a greater dynamic response from the controller.
NOx sensors are also known, and enable the direct measurement of the nitric oxide concentration in the exhaust gas from gasoline and diesel engines. In low-emission designs such as SULEV II, NOx sensors in prior art are used for the optimum control and diagnosis of catalytic converters with the aid of the engine controller, and for compliance with OBD requirements when testing a three-way catalytic converter. For example in the manual “Verbrennungsmotor” (“Internal combustion engine”), second edition, Viehweg, page 590, an operating principle for a NOx sensor is described. This design of NOx sensor provides for two chambers, by which in a first chamber the oxygen contained in the exhaust gas is reduced (lean exhaust gas) or increased (rich exhaust gas) by creating a pump current at constant partial pressure. The necessary current is here proportional to the reciprocal of the air-to-force ratio. In the second chamber, NOx reduction takes place at the test electrode. The current necessary for keeping the surroundings of the electrode clear of oxygen is here proportional to the nitric oxide concentration and usually forms the measurement signal.